618 Invasive Stink Bugs and Related Species (Pentatomoidea)
13.3 Phytoplasmas
13.3.1 General Overview of Phytoplasmas
Originally called mycoplasma-like organisms (MLOs), these prokaryotes are classified as Mollicutes
and are related to spiroplasmas and mycoplasmas. Lacking a cell wall, they are thought to have evolved
from gram-positive bacteria, and have the smallest genome among the prokaryotes. Because phytoplas-
mas cannot be cultured in vitro, Koch’s postulates cannot be fully satisfied for phytoplasma diseases
(Weintraub and Beanland 2006). Koch’s postulates, a set of rules for experimental proof of pathogenic-
ity, require that a pathogen be associated continually with the disease and that the organism be isolated
and grown in pure culture. The pure culture then must be used to inoculate a healthy plant, causing
disease symptoms, and the same pathogen must be isolated from experimentally infected plants (Roberts
and Boothroyd 1972). Even though Koch’s postulates cannot be satisfied fully for phytoplasma diseases,
molecular techniques based on 16S ribosomal DNA (rDNA) polymorphisms have aided extensively in
our understanding of phytoplasma diseases and vectors in recent years. Fruit and timber trees, rice,
legumes, maize, and potato are among the crops affected by these obligate plant parasites. Phytoplasma
diseases are characterized by “yellows,” “greening,” stunting, phyllody (formation of leaf-like structures
from floral parts), and overproduction of axillary buds (“witches’ broom” effect) (Bertaccini and Duduk
2009). All phytoplasmas are phloem-limited and vectored by piercing-sucking feeders, especially del-
tocephaline leafhoppers and several families of planthoppers; in addition, two psyllids are known vec-
tors (Weintraub and Beanland 2006). Infection is acquired passively by ingestion from phloem; these
organisms are small enough to pass through pores in sieve elements and are translocated with the flow of
assimilate. Phytoplasmas then move into the gut and hemolymph and eventually into the salivary glands
(Bertaccini and Duduk 2009).
For many years, it was thought that transmission of phloem-colonizing pathogens, presumably via
hemipteran watery saliva, had to involve non-destructive, selective feeding and hence relatively small
insect mouthparts, as acquisition and especially inoculation were assumed to occur only in undamaged
sieve tube cells (Weintraub and Beanland 2006). However, a lace bug was shown to transmit the coconut
root wilt phytoplasma (Mathen et al. 1990), and these insects typically feed by rupturing cells rather
than selective feeding in the phloem. Subsequent discovery that a relatively large coreid bug was the
vector for a phloem-limited bacterium causing cucurbit yellow vine disease (Pair et al. 2004), and that a
pentatomid (see below) transmitted the phytoplasma of Paulownia witches’-broom (Okuda et al. 1998),
has called this assumption into question. Recently, a phytoplasma associated with coconut lethal yel-
lowing in Mozambique has been found in Platacantha lutea (Westwood), and this pentatomid has been
proposed as a potential vector (Dollet et al. 2011). More studies clearly are needed to fully understand
the transmission of phloem-limited prokaryotes by true bugs.
13.3.2 Empress Tree
The Empress tree, Paulownia tomentosa (Thunb.) Sieb. and Zucc. ex Steud., is native to eastern and
central China where it serves as an important timber tree and for urban landscaping. Planted extensively
in Japan and Korea for timber and on other continents for economic or ornamental purposes, it has
escaped cultivation and become moderately to severely invasive in some regions of the United States
and is considered potentially invasive in Australia (Hiruki 1999, Innes 2009). In East Asia, wood quality
is severely reduced by a phytoplasma infection known as Paulownia witches’-broom disease. Infected
shoots bear sterile, virescent flowers, leaves are faded and distorted, root growth is reduced, phloem
necrosis is evident, cells of the wood are disarranged, and saplings are stunted and killed. Up to 70% of
mature orchard trees may be infected in China and this phytoplasma is the most important factor affect-
ing empress tree timber production in Japan.
The disease may be propagated vegetatively, but the confirmed insect vector is the polyphagous pen-
tatomid Halyomorpha halys (Stål) (Hiruki 1999, Okuda et al. 1998). Some reports in the literature men-
tion H. mista (Uhler) or H. picus (F.) as vectors, but the former is a junior synonym and the latter resulted
from confusion with an Indian species, so these observations now are considered to refer to H. halys in